I just set my drones maximum height at 119m and forget it. If I go over the edge of a cliff (Im in deep sh*t… I mean the drone 
not actually me) and go no lower and less than 120m from its edge I think nothing of it. I’m assuming airdata would show it way less than 120m if I went lower than the edge. 
Anything less than -120m relative to me over 120m out from the edge would indicate I"m flying too high from the ground/water I guess.
it would as it’s based upon the takeoff point… hence if you take off in the valley you will be fine always… but at the top of the hill will need to compensate downwards.
I know the drone fly safe / cap 722 maximum height is 120m but this is not a target but a maximum and it’s purpose is to provide a degree of separation between manned and UAV operations (VFR + IFR)
the CAP 722 guidance also has an image that is very similar to the EASA one
remember… manned aircraft SHOULD: (VFR):
(f) Except when necessary for take-off or landing, or except by permission from the competent authority, a VFR flight shall not be flown:
(1) over the congested areas of cities, towns or settlements or over an open-air assembly of persons at a height less than 300 m (1000 ft) above the highest obstacle within a radius of 600 m from the aircraft;
(2) elsewhere than as specified in (1), at a height less than 150 m (500 ft) above the ground or water, or 150 m (500 ft) above the highest obstacle within a radius of 150 m (500 ft) from the aircraft
Simpler but unrealistic to actually put in practice if your flying from the top of a peak or ridge.
You would have to descend as you fly out when you really don’t have to.
If you fly out 100-120m horizontally at take off height you are all good no matter what the drop below is.
All depends on the situation and location though and wouldn’t really be a problem for the majority.
That’s the image that was used in drone code few years ago.
@callum Thats 100 _ 120m horizontally from the cliff edge isn’t it? Then all measurements would be a minus straight down until the drone reached the land/sea level as long as your inside 120m from the edge. Over the 120m from the edge the height would be taken as a positive below the drone to land/sea level. So if the drone was flying at 120m before going over the edge of the cliff and it went 121m out from a 130m high cliff the drones actual height would be 130 + 120m. Losing the will to live here! 
Your overthinking it, just go and fly no one is checking 
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Who says …
Its a curve over the edge, but other than that, spot on
@ximi Yeh, I thought about doing the curve but TBH I couldn’t be bothered to draw the arrow lines in and the kettle had finished boiling!
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Except @Challenges_Committee members